Centrifugal Filter

ABSTRACT

Filtration device suited for concentration of liquid samples, particularly biomolecules, and a method of concentrating, desalting, purifying and/or fractionating liquid samples. In certain embodiments the device includes a housing having a sample reservoir, and two substantially vertically oriented and spaced apart membranes disposed in the housing. An underdrain is associated with each membrane such that fluid passing through each membrane flows through a respective underdrain into a filtrate collection chamber. The fluid that does not pass through the membrane is collected in the retentate collection chamber, and can be recovered such as by a reverse spinning step, achieving recoveries greater than about 90%. The substantially vertical orientation of the membranes increases the available membrane area by at least 2.7 times the area available in a conventional filter device. The two-panel configuration also maintains more available membrane area in use during the last stages of filtration than a one-panel configuration.

This application is a continuation of Ser. No. 11/903,577, fild Sep. 24,2007, the disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Centrifugal filters may be used to separate biological substances suchas an antibody enzyme, nucleic acid and protein for the purpose ofconcentration, desalting, purification, and fractionation. These devicesare most commonly used in centrifugal-separator instruments, which mayconsist of a fixed-angle-rotor configuration or a swing- orvariable-angle-rotor configuration. The speed of the filtering processand the recovery of retentate sample are highly valued by customers.Sample recovery values higher than 85% are usually obtained by removingthe membrane capsule (sample holder) and reverse spinning it in areceiver tube.

Such devices are typically used to concentrate urine, serum, plasma andcerebrospinal fluid. For example, the measurement of specific proteinsin urine can be important for the diagnosis and management of variousdisease states, yet the content of these proteins in urine is often toosmall to be detected without first concentrating the proteins.Conventional devices generally include a housing having a samplereservoir, a filter sealed in the housing so that the sample must pastthrough the filter when subject to a driving force (such ascentrifugation), and a collection chamber for collecting theconcentrated sample.

Examples of some of the devices that are commercially available includeMicrocon™-type devices available from Millipore Corporation, in which acircular membrane is die-cut, positioned, and clamped in place using asilicone gasket. Ultrafree™-type devices are also commerciallyavailable, in which a rectangular membrane die-cut, positioned, andadhesively bonded in place. An underdrain sleeve is press fitted inplace to secure the membrane in place. However, the sleeves may crackand leak, which may be caused by stresses that occur at the knit line.

Another representative device is disclosed in U.S. Pat. No. 5,647,990 toVassarotti. Vassarotti discloses a centrifugal filter device wherein thefilter is arranged in a sidewall of the filtration chamber, and aconcentrate pocket is arranged in a bottom wall of the filtrationchamber. Upon the application of centrifugal force, a force vector iscreated acting on the macromolecules in the sample and sweeping thefilter surface, the causing the macromolecules to be collected in theconcentrate pocket away from the filter surface.

Still another representative device is shown in U.S. Pat. No. 4,722,792to Miyagi et al., which discloses a centrifugal filter wherein a filterfilm is disposed between a sample chamber and a filter chamber in ainclined or parallel manner to an axis of the filter so that regardlessof whether the filter is used in an angle rotor type or swing rotor typecentrifuge, the filter is positioned, during operation, so as to beinclined or parallel to the centrifugal force to reduce clogging.

However, conventional devices suffer from various drawbacks, includinglower than desired recovery, filtration speed, and/or high price. Itwould be desirable to provide a filtration device, such as a centrifugalfiltration devices, that provides improved recovery and fast filtrationtimes at a competitive price.

SUMMARY OF THE INVENTION

The problems of the prior art have been overcome by the presentinvention, which provides a filtration device particularly suited forconcentration of liquid samples, particularly biomolecules, and a methodof concentrating, desalting, purifying and/or fractionating liquidsamples. More specifically, in certain embodiments the device includes ahousing having a sample reservoir, and two substantially verticallyoriented and spaced apart membranes disposed in the housing. Anunderdrain is associated with each membrane such that fluid passingthrough each membrane flows through a respective underdrain into afiltrate collection chamber. The fluid that does not pass through themembrane is collected in the retentate collection chamber, and can berecovered such as by a reverse spinning step, achieving recoveriesgreater than about 90%. The substantially vertical orientation of themembranes increases the available membrane area by at least 2.7 timesthe area available in a conventional MICROCON™ filter device. Thetwo-panel configuration also maintains more available membrane area inuse during the last stages of filtration than a one-panel configuration.Active membrane area is needed the most during the last stages offiltration, when the retentate sample becomes more concentrated andmembrane surface fouling increases, membrane interior fouling increases,and the fluid height decreases. The device housing is preferably made ofa low-cost material, has low specific protein binding qualities, and isdisposable.

A filtration device of this type is typically used in the followingmanner:

-   -   1. Insert the sample-holder reservoir into a vial.    -   2. Pipette solution into the sample-holder reservoir (e.g. 0.5        ml maximum volume), without touching the membrane with the        pipette tip.    -   3. Place the assembly in a compatible centrifuge and        counterbalance the rotor with a similar device.    -   4. Spin the device using the supplier's “Centrifugal Guidelines”        for the correct spin times and speeds.    -   5. Remove the assembly from the centrifuge. Separate the vial        from the sample-holder reservoir.    -   6. Place the sample-holder reservoir upside down in a new vial        without allowing any of the retentate to fall out, then spin the        assembly for e.g. 3 minutes at 1000×G (or pulse briefly) to        transfer the concentrate to the vial.    -   7. Remove the assembly from the centrifuge. Separate the        sample-holder reservoir from the vial. Snap the sealing cap onto        the vial, and store the product in a responsible manner for        later use.

In certain embodiments a filtration device holder such as a vial isprovided, such that the filtration device (sample holder) can bepositioned in one orientation in the device holder for sampleconcentration, and can be positioned in another orientation (inverted)in the device holder for sample recovery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical oriented, cross-sectional side view of a filterhousing in accordance with certain embodiments;

FIG. 2 is an angular oriented, cross-sectional front view of a filterhousing in accordance with certain embodiments;

FIG. 3 is an isometric oriented, exploded view of a filter housing inaccordance with certain embodiments;

FIG. 4 is an isometric view of the interior surface of a side panel of afilter housing in accordance with certain embodiments;

FIG. 5 is an isometric view of the exterior surface of the side panel ofFIG. 4;

FIG. 6 is a vertical oriented, cross-sectional view of a side panel ofthe filter housing in accordance with certain embodiments;

FIG. 7 is a vertical oriented, cross-sectional view of a device holderor vial in accordance with certain embodiments;

FIG. 8 is an angular oriented, cross-sectional view of a device holder,which shows a filtration device containing the maximum volume of aretentate fluid and filtrate fluid in a spin mode in accordance withcertain embodiments;

FIG. 9 is an angular oriented, cross-sectional view of a device holdercontaining a filtration device in a reverse-spin mode in accordance withcertain embodiments;

FIG. 10 is a schematic diagram showing a position of the device holderduring the molding process;

FIG. 11 is an isometric view of a textured underdrain design inaccordance with certain embodiments;

FIG. 12 is an isometric view a textured underdrain design havingdiagonal-oriented flow channels in accordance with certain embodiments;

FIG. 13 is an isometric view of a textured underdrain design havingraised cylindrical protrusions, which form flow channels in accordancewith certain embodiments;

FIG. 14 is an isometric view of a textured underdrain design havingraised polygon protrusions, which form flow channels in accordance withcertain embodiments;

FIG. 15 is a plot of the results of water flow performance testing of adevice made using five different membrane-device configurations (3 kDaMWCO, 10 kDa MWCO, 30 kDa MWCO, 50 kDa MWCO and 100 kDa MWCO) inaccordance with certain embodiments;

FIG. 16 shows plots of the water retentate volumes for 30 kDa MWCO and50 kDa MWCO devices and comparable Microcon™ devices as a function ofspinning time at 14,000 Gs;

FIG. 17 is a plot showing that the typical values of airflow leakage foracceptable devices made with all five membrane-device configurationswere less than 0.35 cubic centimeters (cc) per minute;

FIG. 18 is a plot showing that the overmolding process successfullybonded the two panels into a single device in a manner that demonstratedacceptable pressure integrity;

FIG. 19 is a plot showing that the levels of average protein passage(proteins that should not pass) were less than 5%, which is consideredto be an acceptable level of performance;

FIG. 20 is a plot showing that the average protein that was recoveredwas greater than 90% for the devices that were tested, which isconsidered to be an acceptable level of performance;

FIG. 21 is a plot showing that the average total volume of sample thatwas recovered for these devices was greater than 98% when the reversespin procedure was used;

FIG. 22 is a plot showing that the results of the average dead-stopvolume that were measured when these devices were spun for 10 minutes;

FIG. 23 is a bottom view of the overmolded jacket, which describes therelative proportions of the wide flange feature compared to the overallgeometry of the sample holder;

FIG. 24 is a side view of the overmolded jacket, which describes therelative proportions of the wide flange feature compared to the overallgeometry of the sample holder;

FIG. 25 is a side view of sample holder, which describes location of thedrain holes and the space needed between the wide flanges to ensureadequate drainage of filtrate during spinning operations;

FIG. 26 is an isometric view of the core pin used as part of the processto overmold two panels into one device; and

FIG. 27 is an isometric view of the heat seal nest, which includesalignment slots for the panel.

DETAILED DESCRIPTION OF THE INVENTION

Turning first to FIG. 1, there is shown filter device 10. The device 10includes a sample reservoir 11 to receive an unfiltered sample, andfirst and second membranes 12A and 12B each arranged on a side wall ofthe device 10 as shown. A retentate chamber 14 defining a dead-stopvolume is provided below the membranes 12A and 12B. A collection tip 30(FIG. 2) that is generally arc-shaped and protrudes outwardly from thebottom perimeter of the device may be provided to localize the dead-stopvolume at the centerline of the device, and subsequently reducevariability of the dead-stop volume as the angle of orientation in acentrifuge changes. Preferably the device 10 is made of a solid materialthat is liquid impermeable, has low protein binding characteristics, andis sufficiently strong to withstand the gravitational forces (Gs)applied during centrifugation. Suitable materials include acrylic,CYROLITE G20 HiFlo resin, ESTAR HN631 resin and KRATON polymers. Theside panels 15A, 15B (FIG. 3) in particular can be made of a clearplastic material which enables an operator or user to see into theinterior cavity of the device so as to determine the fluid levels priorto, and after the filtration process.

Preferably the device 10 is formed by attaching two flat sheet membranesto side panels 15A, 15B (FIG. 3), which are then overmolded into adevice housing. Side panel 15A (FIG. 4) includes an underdrain support16 that supports the membrane and provides fluid communication to theretentate chamber 14. For example, the underdrain support 16 can includea series of spaced longitudinal grooves, channels, or surface texturesthat are located beneath the membrane to capture filtrate as it passesthrough the membrane and direct it towards the drain holes and into areceiver vial. Side panel 15B is similarly configured. Each membrane issealed to a respective side panel 15A, 15B so that only fluid passingthrough the membrane can exit the drain holes of the device located inthe side panels.

In certain embodiments, each membrane 12A, 12B is coextensive with arespective underdrain support 16 and is sealed thereto. The geometry ofthe underdrain is intended to support the membrane and keep it as flatas possible, while allowing sufficient open space underneath themembrane to enable fluid to flow and pass through the drain holes 18 ofthe device. It is preferred that hydraulic fluid resistance be kept aslow as possible.

FIGS. 11,12,13 and 14 show examples of suitable textured surfacepatterns of the underdrain, such as raised cylindrical protrusions 16(FIG. 13) or multi-faceted columns that protrude up from the back sideof the panel. The top surface of the protrusions contact and support themembrane at intervals, which minimize in-plane stretching of themembrane during the high pressures that are generated during centrifugalspinning operations. These also support the membrane and allow fluids toflow underneath the membrane and out through the drain holes 18. Whenthe underdrain geometry inadequately supports the membrane, the pores ofthe membrane may stretch, and become elongated. These enlarged poressizes then compromise the retentive characteristics of the membrane. Theembodiment of FIG. 11 is a series of raised irregular shapes 16. Theembodiment of FIG. 12 is a criss-cross pattern forming raisedparallelograms 16. The embodiment of FIG. 14 is a hexagonal pattern ofraised protrusions 16 that may also be used to form an underdrainstructure. Those skilled in the art will appreciate that these patternsare examples only and that other suitable patterns are within the scopeof the present invention.

Suitable membranes include microporous and ultraporous membranes, thelatter being useful for ultrafiltration. Regenerated celluloseultrafiltration membranes (e.g., “Ultracel Amicon YM” and “Ultracel PL”membranes available from Millipore Corporation of Bedford, Mass.) arewell-suited for devices targeted for concentrating or desaltingextremely dilute or hydrophobic sample liquids. The use of a hydrophilicmembrane having a “tight” microstructure promotes good retention withlow adsorption of protein, DNA, and other macromolecules.Polyethersulfone ultrafiltration membranes (e.g., “Amicon PM” and“Biomax PB” also available from Millipore Corporation), or other likemembrane having an “open” microstructure suitable for rapid separation,are better-suited for devices targeted for concentrating and desaltingmore concentrated sample liquids, such as serum, plasma, and conditionedtissue culture.

Preferably each membrane 12A, 12B (FIG. 1) is oriented at a slight anglewith respect to the longitudinal centerline of the device 10, such thatthe top of each membrane is spaced from the longitudinal centerline adistance greater than the bottom of the membrane. A funnel-shapedconfiguration is formed. So positioning each membrane takes advantage oftangential flow effects during centrifugation. An angle greater thanabout 0° and less than about 5°, preferably about 3°, has been found tobe suitable.

The tapered, side-by-side two membrane design also has an inherentself-cleaning feature, reducing the amount of membrane fouling duringcentrifugation.

The tapered, side-by-side design also enables pipette tips to easily fitinto the sample holder from the top and reach down to the bottom of thedevice. Typically a user of this device would use: a) pipette tip toextract the desired retentate volume that has accumulated in the bottomof the device, or b) reverse spin the device in a vial or similarholder. The space at the bottom of the device which contains theretentate sample is usually referred to as the dead-stop volume.

As seen in FIGS. 3 and 5, each of the side panels 15A, 15B includes oneor more drain holes 18 that are in fluid communication with theretentate chamber 14 and enable filtrate to pass through the devicehousing 10 for collection in another housing such as a vial 75 (FIG. 7).In the embodiment shown, four such drain holes are illustrated in eachof the two panels, although the invention is not to be so limited. Eachdrain hole 18 is preferably located at the bottom of a respectiveunderdrain groove or channel and is preferably substantially circular incross-section. The drain holes should be located a sufficient distancefrom the side edges of the panels 15A, 15B so that the holes are notconstricted or otherwise deleteriously altered during a heat sealoperation that can be used during manufacture of the device. Preferablythe drain holes 18 are equally spaced from one another and areco-linear.

The side panels 15A, 15B are preferably identical and are configured tosnap fit or press fit into one another. Preferably a labyrinth seal isused to seal the side panels to one another. To that end, an exemplarylabyrinth seal is a rib-in-groove configuration, wherein each side panelincludes a groove 19 formed along one inner edge thereof, and a rib 20formed along the other inner edge thereof (FIG. 4). The groove 19 andrib 20 are configured such that when side panel 15A is brought intoalignment with side panel 15B, the rib 19 of side panel 15A and thegroove 20 of side panel 15B engage, as do the rib 19 of side panel 15Band the groove 20 of side panel 15A, to lock the side panels together.This labyrinth configuration helps to ensure that the panels aresymmetrically aligned prior to overmolding and throughout theovermolding process; it assists in retention onto a mold core and thusfacilitates automated assembly and makes the assembly process moreresistant to machine faults. The labyrinth alignment also functions as aplastic flow trap, preventing the overmolded material from ingressinginto the sample volume. The groove 19 can include one or more portionswith a deeper recess, each of which receives a corresponding portion onthe groove with a higher protrusion, to facilitate the snap-fit betweenthe two panels. Those skilled in the art will appreciate that otherdesigns to create a snap fit or press fit and a seal are within thespirit and scope of the invention, such as a series of spacedprotrusions mating with a series of spaced wells.

Alignment dowels 86 and 87 FIG. 3) are integrally molded into theexterior surface of each panel to enable panels to precisely registerinto an alignment hole in the nest fixtures that are used to hold thedevice in place during membrane attachment and automatedpick-and-placement. An automated process can be used to move the panelsinto a membrane attachment module, membrane alignment module, overmoldmodule, and in-process quality inspection modules. One of the dowels isdesigned to fix the panels in place in a nest 108 (FIG. 27), while asecond dowel 87 allows the panel to grow in one direction due to thermalexpansion effects that may occur during processing. The dowels alsoprovide a finger-grip feature that enables operators to comfortably holdthe device while wearing elastomeric gloves that may have been treatedwith water or a cleaning solvent, and may still be wet during handling.The protruding dowels also help prevent the device from accidentallyslipping out of a user's hands.

FIG. 3 shows that an overmolding process is used to manufacture thedevice 10. The side panels 15A, 15B may also include a top molded flange17 and side flanges 110 that helps mechanically capture the overmoldedjacket onto the panels. The flange 17 and flange 110 form an anchorsthat are used to secure the panels to the overmolded plastic thusforming an integral and robust device. This improves resistance tohoop-stress failures and device-burst pressures. Indeed, the said devicehas been shown to withstand pressures greater than about 250 psi (FIG.18), well above the 30 to 80 psi necessary for effective operation.Preferably the panels are made of a polymer that has a higher melttemperature than the melt temperature of the overmolded material. Thesematerials may include: polystyrene, acrylics, styrene butadienecopolymers, styrene acrylonitriles, CYROLITE G20 HiFlo resin, ESTARHN631 resin and KRATON polymers. These materials enable the over moldingprocess to be more robust such that thermal heat transfer into thepremolded material does not lead to thermal distortion of the premoldedparts geometry, which compromises the device's overall form, fit, andfunction.

The over-molded jacket 111 (FIG. 3) may include an integrated wideflange 88 to help keep the cross-section of the walls of the vial in acircular shape during centrifugation. The jacket attaches the two panelstogether by a combination of thermal fusion of the thermoplasticmaterials and mechanical interlock of the panel's flanges 17 and flanges110. The jacket begins at the top of the sample holder 89 (FIG. 1),where fluids are added to the device and where a vial cap 80 (FIGS. 7 &8) is used to close the device such as during centrifugal spinning. Theinner diameter of the jacket 89 (FIG. 1) is formed in a continuousfashion around one core pin 102 (FIG. 26) which helps ensure acceptabledimensional control of the cap seal 81 (FIGS. 7 & 8). A split partingline was used on the outer surface 91 (FIG. 1) along the device'slongitudinal axis, but was not used along the inner diameter in order toprevent “parting-line misalignments”. Parting line misalignments usuallyoccur when molds are designed to open in two halves. Eliminating thesplit-parting line on the inner diameter of the overmold helps ensurethat undesirable gaps between the cap and the device do not occur. Theabsence of these gaps help to ensure the acceptable form, fit andfunction of the fluid seal at the sample holder and vial cap.

The jacket includes an overmolded material that maintains the samediameter from the top 112 of the sample holder (FIG. 2) to the bottomedge 113. The bottom edge of the jacket includes a integrated wideflange 88 (FIG. 3) that mechanically contacts the inner diameter of thereceiver vial 85 (FIG. 7), and holds up the sample holder in the vial.This feature facilitates high speed spinning operations in a centrifugeby distributing the loads or stresses generated by the device into theshoulder of a receiver vial.

FIGS. 23 and 24 show the relative proportions of the integrated wideflange feature 88 compared to the overall geometry of the sample holder.The side view shows that the width of the wide flange feature 88 (FIG.24) (e.g., 0.250 inch) is wider than the width of the overmold seal 115(e.g., 0.144 inch), which fuses the two panels together into one device.The width of the wide flange 88 needs to be at least 65% of the outerdiameter of the sample holder 116 (FIG. 24) to ensure that stresses areadequately distributed and transferred to the support rim of the vial 85(FIG. 7). The width of the wide flange 88 should not be more than least80% of the outer diameter of the sample holder because there must beenough space 117 (FIG. 25) for filtrate to drain out of the drain holes18.

The transition 118 of the wide flange feature 88 (FIG. 25) begins at alocation on the overmold seal 119 which is approximately equal to onehalf the diameter of the device. The transition curves from the overmoldseal are intentionally gradual so as to minimize stress concentrations.The outer diameter of the wide flange feature 88 is the same as theouter diameter at the top rim of the sample holder. This feature enablesthe sample holder to be inverted and spun in the same receiver vial asshown in FIG. 9.

The wide flange 88 provides enough material support to keep the devicein place during centrifugal spin operations that may be equal to andgreater than 10,000 Gs. Experiments have shown that the flared featureenabled devices to be spun at centrifugal spin loads as high as 16,000Gs for one hour without damage. When the flared feature was not used,some devices plastically deformed and collapsed into the bottom of thereceiver vial. Failure analysis of devices showed that the flaredgeometry was needed to keep the walls of the vial as circular aspossible, and also distribute the contact stress between the device andthe support rim 85 of the vials below the plastic yield stress of thevial.

The wall thickness of the overmolded wall at the top of the device 89(FIG. 1) needs to be sufficiently thick in order to prevent splittingfractures due to stresses that are generated by the vial cap 80 duringcentrifugal-spin processes. A suitable thickness is 0.044 inches. When avial cap 80 is pressed onto the sample holder to establish a liquidseal, hoop stresses are generated. When devices are spun in a centrifugeat speeds that generate 16,000 Gs or more, the mass of the cap combinedwith the snap-fit feature create tensile-hoop stresses in the sampleholder. If these stresses are high enough, the side wall of the sampleholder fractures along the knit line. The knit line refers to the jointwhere the two or more melt flows of plastic meet and are fused togetherduring the overmold process.

The overmolded jacket 111 (FIG. 3) may be made using different colorantadditives which enable different device configurations to bedifferentiated by means of color.

In order to ensure that devices do not fracture, the wall of the sampleholder must be thick enough to prevent the: a) elastic deformation thatwould enable caps to open—which is undesirable, and b) plasticdeformation and rupture that would allow leakage of the samplefluid—which is also undesirable. For the low-protein-binding material ofchoice styrene butadiene copolymer—a wall thickness of at least 0.035inch was found to be suitable.

When the wall thickness 89 (FIG. 1) is increased, the internal volume(FIG. 1) of the sample holder is reduced to undesirable levels. Samplevolumes of less than 0.45 ul are commercially undesirable. Having adevice that has a sample volume up to 0.5 ul is considered to bedesirable and of strategic commercial value.

In some test cases, increasing the temperature of the mold base from 90°F. to 125° F. was used during overmolding to more effectively fuse theknit lines together and achieve greater strength. Care must be taken toensure that any additional heat used does not cause the bottom of thepanels to melt and collapse—which is undesirable.

The two panels of the device need to be thick enough and stiff enough tosupport the overmold pressure at the nose of the panels and at thecenter of the panels. Experiments using current geometry andstyrene-butadiene material revealed that a wall thickness of at least0.0585 inch was needed to prevent unacceptable deformation and collapse.This thickness and a suitable wall strength was needed even though thepanels were supported by a steel-core pin 102 (FIG. 26) during theprocess of overmolding panels into the completed device. The surface ofthe pin that is closest to the membrane surface 103 was relieved toensure that the membrane never comes into contact with the core pin 102(FIG. 26). The retentive layer of the membranes can be damaged when themembrane comes into contact with core pins and can be scratched as theparts are ejected from the overmold.

To prevent membranes from being pulled away from panels and scratched bythe surface of the core pin 102, the pin was fabricated with vent holes104 that enable air from the mold cavity to be evacuated through thecenter of the core pin. This unique core pin design enables theovermolding of devices in a manner that does not over pressurize andblow the attached membrane off of the panels.

Special attention must be given to the design of cooling ports in theovermolded cavity. During some experiments it was observed that when thetemperature of the overmold material, as measured by the temperature ofthe base mold at 90° F., was increased by only 35 degrees F. to 125° F.,the additional heat that flowed into the panels caused them to beginmelting and collapsing onto the core pin. In the most extreme cases thepre-molded panels deformed enough to completely close off the drainholes. In some cases the heat effects were sufficient to allow panels toslightly move away from the overmolded cavity and allow plastic leak andto flow over the exterior wall of the panel. In some cases the leakagewas small enough that the devices were still of good quality. In theworst cases the plastic flowed all the way up to the drain holes andpartially filled them. This was considered to be undesirable because theflow through the drain holes was restricted.

The adverse affects of heat on the pre-molded panel can be overcome byimproving thermal cooling of the mold cavity and core pin, and by usinga valve gate at the plastic injection port 92 (FIGS. 1 & 23) instead ofusing narrow-edge gates. Narrow edge gates create high levels of shearflow in the plastic, which generate more heat. These shear flows can bereduced by using a valve gate, which has a larger cross-sectional flowarea. This increased area reduces shear heating affects and enables theovermolded cavity to be more easily filled.

Valve gates are using in injection molds to direct the flow of meltedplastic polymer from the hot runner into the mold cavity. To achieve thebest molding results, this flow of polymer should be directed towards asolid surface in the mold cavity such as a core pin 95 (FIG. 10) and 105(FIG. 26). This flow needs to be broken up into turbulent swirls toprevent material flow marks and jetting, which could roughen the surfaceof finished parts.

The adverse affects of overmolding heat can also be overcome by placingcooling lines closer to the part's surface and by including thermalcooling lines in the core pin. This can usually be accomplished by usingtypical cooling fluids such as water or propylene glycol solutions.

The adverse affects of heat can also be overcome by using mold insertsthat that have higher values of thermal conductivity. Materials thathave a higher thermal conductivity will enable heat to be drawn awayfrom parts more effectively than when materials having a lower thermalconductivity are used. These inserts are usually fastened into the moldcavity, and help transfer heat away from a pre-mold more effectivelythat when one type of steel is used. Typically inserts can be made usingmetals, such as beryllium, copper and aluminum.

FIG. 10 shows how leakage that may occur in the nose of the panel can beovercome by adding a protrusion 93 on the outer surface of the panel,which acts as a seal. This seal feature can be integrally molded intothe panel's outer surface and functions like a mechanical O-Ring seal.When the overmolded cavity is closed over the panels, the cavity wallmakes intimate contact with the integrally-molded-seal feature. Thisseal helps prevent leakage of plastic into the drain holes 18 of thesample holder.

Preferably the core pin 102 (FIG. 26) is made of a harden steelmaterial. Although using a core pin made of P20 steel, hardened to aRockwell hardness of 32-34 Rc, may be adequate to successfully make asmall number of devices, the core pin may ultimately deform, whichcompromises the housing-burst strength of devices as measured byhousing-burst pressure. Accordingly, preferably the core-pin material isan H13 steel hardened to a Rockwell hardness of 53-54 Rc, which is moredurable. Thus, the material hardness and geometry of the core-pin designneed to be carefully controlled to successfully make devices on acommercial scale.

It is also very important to control the following factors during theovermolding process so as to ensure that a good device is made:

A. The design and shape of the core pin must hold the panels securely insuch a way that the core pins surface 103 (FIG. 26) does not come intocontact with and damage the membrane's retentive surface.

B. The design, shape, and selection of materials for the core pin mustensure that adequate cooling is achieved during continuous moldingoperations. An inadequate cooling will result in thermal deformation ofpanels.

C. The design, shape and selection of materials for the core pin mustalso ensure that undesirable deflections do not occur doing continuousmolding operations. Stiffness was successfully achieved by includinginto the core pin's design support grips 106, which stiffened theoverall design and reduced lateral deflections. The end 105 of the corepin 102 must be strong enough and stiff enough to push the nose of thepanels into the overmolded and properly seat the panels into the moldcavity 96 so as to prevent the panels from being pushed away from theinjection-valve gate 97 during overmolding.

D. The clearance 98 (FIG. 10) between the support alignment dowels inthe premolded panels and the recess 99 in the overmolded panel must beminimized. This is needed to prevent the panels from moving away fromthe injection-valve gate during over mold operations.

E. The clearance gap 100 (FIG. 10) between the premolded panels and overmold cavity must be carefully controlled and kept to a minimum in orderto prevent undesirable leakage of the overmolded material along theexterior wall of panels.

F. The dimensions of the seal interface between the panels and theovermold cavity must be very carefully controlled to ensure goodshut-off qualities, which will prevent plastic leakage into the drainholes 18 (FIG. 3) and into the membrane area 12A and 12B (FIG. 1).

G. Adjusting the process parameters used for overmolding may provideincremental improvements toward preventing undesirable leakage ofpolymer material across the seal interface between panels and theovermold cavity. These process parameters consist of: a) resin stoptemperature during molding, b) temperature of the mold base, and c) useof back pressure during molding. However, making adjustments to theseprocess parameters is not a substitute for properly dimensioning theseal interface between panels and the overmold cavity.

H. The size and shape of the core pin must ensure that there issufficient clearance between the surfaces 103 (FIG. 26) of the core pin102 and the membranes 12A and 12B (FIG. 1) that are attached to thepanels, so as to ensure that the two do not come into contact during theovermolding process. Any contact may result in serious damage to theretentive layer of the membrane, which may compromise the overallperformance of the device.

I. The size and shape of the core pin must also ensure that there is arecess pocket 107 (FIG. 26) with sufficient clearance at the tip 105 ofthe core pin 102 to support the dome feature 25 (FIGS. 1 & 6) that isintegrally molded into the tip of each panel.

To make good pre-molded panels, an injection mold valve gate needs to bepositioned at the nose 92 (FIG. 23) of the panels near the drain holesto ensure the best structural integrity of the overall device. Placingthe valve at the nose of panels enables plastic material flow into themold and preferentially positions the material knit lines in theunderdrain structure, and not across the area where the membrane isattached. Allowing the knit lines to occur at the membrane attachmentsite compromises the devices retention performance. Heating processesused to attach the membrane material can cause the knit lines to open inan undesirable way, which allows fluids to leak around the seal.

The membrane coupons are die-cut using an automated, matched die set inorder to achieve the coupon-to-coupon dimensional accuracy that isneeded. The process of automated die cutting, pick-and-placement, andheat sealing of coupons is very important to the manufacture of thesedevices. Automated processes help reduce surface damage that can occurto the retentative layer of the membrane coupons. Automated processingalso helps to reduce the labor content of manufacturing these devicescompared to using manual manufacturing processes. The adverse effects ofoperator to operator variability are also reduced when automatedmanufacturing processes are used.

The top edge 89 (FIG. 1) of the sample holder device should bespecifically designed to form a continuous rim during overmolding. Theinner diameter 90 of the sample holder must be formed on one core pin102 (FIG. 26), which ensures that one smooth and consistent sealingsurface is formed. This ensures that a good sealing qualities areachieved between the sample holder and the vial cap. Since theovermolded design requires that a split mold can be used, the partingline for the mold halves should be positioned 90 degrees away from theplastic knit lines. This design helps to prevent the alignment ofresidual molding stresses, knitlines, and parting lines. This featureenables the samples holder to sustain higher stresses during centrifugalspinning. Higher stress capabilities enable higher spin speeds to beused, which enable shorter filtration times to be achieved. Thesecombined features provide a unique value proposition to customers.

A labyrinth seal feature 19 and 20 (FIG. 4) is specifically designedinto the edge of each panel to: a) facilitate alignment of the twopanels 15A and 15B (FIG. 3) during assembly, and b) control and preventthe undesirable ingression of plastic during overmolding. As mentionedabove, an exemplary labyrinth seal feature includes a raised geometry onone side of the panel's center line 20 (FIG. 4) and an identicalrecessed geometry 19 (FIG. 4) on the other side of the centerline. Theshape of this feature is preferably symmetric about the center axis ofthe panels, which enables one mold cavity to make the two panels thatwill be assembles into one device. This helps reduce the cost of havingto mold two separate panels to achieve the same assembly. The sealfeature also creates a tortuous path between the inner volume of twoassembled panels and the outer space surrounding the panels. Thetortuous path helps seal off the edge of the panels when the overmold isclosed, which enable the overmold plastic to flow and seal the twopanels together. This tortuous path helps prevent the overmold plasticfrom flowing into the inner volume of the sample holder.

The shape and location of the drain holes 18 on the panels werespecifically designed to help achieve the low variations in dead-stopvolumes, acceptable fluid flow, and acceptable mold durability. The corepins used to form the drain holes were designed with a 5 degree draft oneach side. This draft improves the pin's strength and enable the pins toeasily separate from the molded panels. The draft also creates a taperedhole, such that the more open side of the hole was placed on the innerportion of the panels. The drain holes are still substantially circularin cross-section. The hole narrows as fluid moves out toward thedischarge surface and out of the device. The tapered hole helps improvethe fluid flow through the drain holes by making use of vena contractaaffects.

Vena contracta effects describe laminar fluid flow through an orifice,where Bernoulli's equation predicts that the cross-sectional area of thefluid stream narrows or necks down as a Newtonian fluid passes throughan orifice. Fluid flow becomes more efficient when the surface of thehole follows the streamline of the fluid flow. Less flow separationoccurs, which reduces occurrences of recirculating-fluid flows. Reducingrecirculating fluid flows can help reduce the formation of entrained gasbubbles in the fluid stream, which can lead to foaming. Foaming of thefiltrate in the receiving vial is undesirable because it can cause fluidto wick back into the sample holder when the centrifuge stops.

The retentate chamber 14 (FIG. 4) defines a dead-stop volume whereretentate sample is collected and can be recovered. Preferably theretentate chamber includes a three dimensional discontinuity thatprotruded out of the interior wall, such as a mound-shaped protrusion 25(e.g., FIGS. 4 & 6), to help localize the dead-stop volume at thecenterline of the device, and reduce variability of the dead-stop volumeas the angle of orientation in the centrifuge changes. Morespecifically, the funnel-shaped configuration at the bottom of thesample holder and the discontinuity assist in localizing the retentatefluid into a smaller space at the bottom of the dead-stop volume.Localizing this fluid into a smaller space facilitates the more completeremoval of fluid from the device when a pipette is used. If thisgeometry were not used in the device, a user would have to move apipettor back and forth across the bottom of the dead-stop volume usingone or more passes. In addition, the funnel-shaped configuration and thediscontinuity also reduce the variation in the dead-stop volume thatoccurs when the sample holder is oriented at different angles in thecentrifuge.

The height and shape of the discontinuity 25, such as a mound-shapedprotrusion, was determined as follows. The sample holder was placed in areceiver tube, and oriented differently in two degrees of freedom. Inthe first of these degrees of freedom, the center-line axis of thedevice was aligned with the orientation of a fixed-angle rotor used incentrifugation. Typically the fixed angles are between 35 and 45 degreesfrom the vertical position. Through trial and error, an optimum heightand shape of the discontinuity was determined which achieved the leastvariation in dead-stop volume.

In the second degree of freedom, the sample holder was placed in areceiver tube and placed in a fixed-angle rotor. Under these conditionsthe sample holder could still be rotated on its center-line axis at aninfinite number of angles between 1° and 360°. Therefore, two of themost extreme orientations were chosen; a) a front orientation where thetwo membrane planes are positioned as vertically as possible, and b) aside orientation where the two membrane planes are positioned as closelyas possible to the rotor angle, which typically may be between 35 and 45degrees. Again through trial and error, a height and shape of theprotrusion was determined which achieves the least variation indead-stop volume.

One suitable height of the tear-drop shape 25 is 0.020 inches.

Table 2 summarizes the results of tests which compared the performanceof a centrifugal device in accordance with certain embodiments of thepresent invention with a conventional Millipore Microcon® device. Twodifferent membrane configurations were considered for this test: 30 kDamembrane, and 50 kDa membrane. The results showed that the presentdevices filtered the water much faster than the Microcon® devices, asexpected because of the increased available membrane area. The plotsshowed that the Microcon® devices continued to filter water until thedryness condition was reached where no water remains upstream of thefilter. The present devices continued to filter water until the designeddead stop volume was reached. These data demonstrate that the presentdevice can be used to filter and concentrate samples to a specific deadstop volume without having to worry about filtering to dryness, whichcan compromise sample recovery.

In certain embodiments, the retentate is recovered using a reverse-spinoperation, wherein the device 10 (FIG. 9) is placed in a device holder75 such as a vial or other suitable housing in an inverted manner, andis subject to a driving force such as centrifugation, forcing theretentate fluid from the retentate chamber 14 and into the device holder75. As best seen in FIG. 9, the device holder 75 is preferablycylindrical in cross-section, with a diameter wide enough to accommodatethe filter device 10 in both a forward-spin mode (FIG. 8) and areverse-spin mode (FIG. 9). Preferably the filter device 10 fits snugglywithin the device holder 75. The device holder 10 can include a shoulder85, which is preferably annular, and serves to form a smaller innerdiameter portion of the holder 75 so as to provide a stop, limiting theextent to which the filter device is insertable into the holder 10 (ineither the spin mode or reverse spin mode). Thus, in the spin mode ofFIG. 8, the top flange 76 of the filter device 10 remains outside of theholder 75, and in the reverse spin mode of Figure, the bottom portion ofthe device 10 remains outside of the holder 75 as shown. When the filterdevice 10 is so positioned in the device holder 75, sufficient volume 77below the filter device 10 should remain to receive sufficient liquidproduct obtained during a reverse spin operation.

The design and shape of the shoulder within the vial is very importantto the form fit and function of the device. The highest levels ofcentrifugal loading can be achieved when a continuous circumferentialring 85 design is used. When a discontinuous shoulder design is used,lower levels of centrifugal loading are possible. This lower capacityreduces the ability to perform filtering operations at higher speeds inthe centrifuge.

The device holder or receiver vial preferably includes a cap 80, whichcan be integrally mounted to the holder 10 as shown. The cap should bedimensioned to both cap the device holder 75 when the filter device 10is not positioned in the holder, and to cap the filter device 10 when itis positioned in the holder in the spin mode as shown in FIG. 8. To thatend, the cap 80 can include a first smaller cylindrical portion 81 thathas an outer diameter slightly smaller than the inner diameter of boththe device holder top and the filter device top, and a second largercylindrical portion 82 that has an outer diameter larger than the outerdiameter of the first smaller cylindrical portion 81. The diameter ofthe larger cylindrical portion 82 (FIG. 9) is preferably still smallenough to fit inside the device holder 75 (FIG. 7), but is large enoughso as not to fit inside the filter device 10 (FIG. 8). Preferably thereis a recessed volume of revolution 90 (FIG. 1) formed into the innerdiameter of the sample holder 10. This recess forms a pocket, whichfacilitates the insertion, retention of, and snapfit of the vial cap 80.

Centrifugation times are not particularly limited, and generally spanbetween about 1 minute and 10 minutes. The filtration device and thedevice holder 75 can be provided to the end-user in a packaged kit form,together with instructions for assembly and use.

In certain embodiments, sample recovery can be accomplished by insertinga pipettor and tip into the housing, and more specifically, into theretentate chamber 14, and removing the filtrate therefrom.

A suitable process for manufacturing devices in accordance with certainembodiments is as follows:

-   -   1) Mold panels in advance.    -   2) Die-cut membrane coupons from membrane roll stock.    -   3) Position coupons onto panels and in place on the panels using        heat, pressure, and time.    -   4) Visually inspect the resulting subassembly to ensure proper        alignment of coupons on the panels, overall quality of the        coupon attachment, and the absence of dirt and foreign debris.    -   5) Place the panel-subassemblies into the A 96 (FIG. 10) and B        (FIG. 10) cavities of the over mold.    -   6) Close both the A and B sides of the over mold so as to bring        the panels into contact and seal the labyrinth edge of the        panels.    -   7) Insert the core pin into the overmold cavity in such a manner        that: a) the panels are pushed into the bottom of the over mold        and fixed in place, b) the attached membrane coupons are not        damaged, and c) the dimension requirements of the finish device        are maintained.    -   8) Inject the over mold plastic into the mold cavity.    -   9) Open the over mold and withdraw for device from mold cavities        in A and B.    -   10) Eject the device from the core pin.    -   11) Verify the integrity of the membrane attachment process        using a gas-pressure-decay instrument.    -   12) Transport devices to an instrument and successfully pad        print graduation marks onto the side of the device.    -   13) Package the device into a suitably designed flexible pouch        and carton.

A suitable process to concentrate samples using the device includes thefollowing:

-   -   1) Remove the sample holder device and receiver vial from their        packages.    -   2) Insert the sample holder device into a receiver vial.    -   3) Pipette the sample solution into the sample holder reservoir        (e.g., 0.5 ml maximum value), without touching the membrane with        the pipette tip. Pull cap that is attached to the receiver vial        over the sample holder device, and snapfit into place so as to        ensure a good seal between the cap and the sample holder.    -   4) Place the sample holder device and reservoir into a        compatible centrifuge and be careful to counterbalance the rotor        with a similar device.    -   5) Spin the device in the centrifuge using the supplier's        “Centrifugal Guidelines” for the correct spin times and speeds        for the application.    -   6) After completing this spinning process, remove the sample        holder and reservoir from the centrifuge. Separate the vial from        the sample holder device by lifting it up and out of the        reservoir without spilling the contents of the sample holder or        reservoir.    -   7) Place the sample holder reservoir upside down in a new        receiver vial without allowing any of the retentate volume 14 to        fall out or be lost. Spin the sample holder and vial assembly        for three minutes at 1000×Gs (or pulse briefly) to transfer the        concentrate to the vial.    -   8) Remove the sample holder and vial from the centrifuge.        Separate the sample holder device from the vial. Snap the cap on        to the receiver vial, and store the product in a responsible        manner for later use.

FIG. 15 and Table 1 show the results of water flow performance testingusing the aforementioned filtering process. These results demonstratethat the devices successfully filtered the water sample as intended, andconcentrated the sample to a specific dead-stop volume. The devices donot allow the sample to filter to dryness. Instead of filtering todryness, the sample is concentrated into the dead-stop volume at thebottom of the sample holder device and the retentate can be removedusing a pipette or a reverse spin operation. The data also show that themore open membrane structures, such as the 100K devices, have a fasterflow rate than devices having less open membrane structure such asdevices containing membrane having 3 kDa MWCO.

The ability to concentrate sample solutions to a known concentrationvolume or dead stop volume without the risk of drying the sampledelivers important value to users. Using this device, customers are ableto concentrate their samples in a robust manner which does not requirethem to continuously monitor the concentration process. Essentially,they can turn on the centrifuge for a specified period of time, walkaway, and know with confidence that their samples will be concentratedas desired. When other devices are used which do not have a dead stopfeature, users take the risk of drying out their samples which cancompromise the usefulness of the sample and the subsequent analyses.

TABLE 1 Data that supports FIG. 15, which demonstrated water flowperformance of Amicon Ultra 0.5 ml device made at PTI, using membraneshaving five different (3 kDa, 10 kDa, 30 kDa, 50 kDa & 100 kDa) valuesof molecular weight cutoffs (MWCO). MCWO Time Avg Ret St Dev 3,000 1 3737.5 2 281 19.6 4 173 5.1 8 96 7.7 12 71 6.0 20 42 2.6 30 26 1.3 10,000 1236 5.3 2 136 5.0 4 76 8.6 8 38 2.6 12 22 0.8 20 14 2.6 30 8 1.0 100,0001 46 1.8 2 31 2.5 4 27 9.5 8 21 4.3 12 16 0.5 20 17 2.9 30 14 4.1 30,0001 128 6.7 2 66 1.2 4 40 1.2 8 24 1.0 12 18 1.3 20 16 0.8 30 14 1.350,000 1 83 2.4 2 43 4.5 4 29 1.9 8 19 1.0 12 17 0.5 20 15 0.5 30 14 1.0

These results demonstrate that the vast majority of the devices thatwere made delivered the levels of performance that were targeted. Inmost cases the recovery of protein was greater than 90%. In the caseswhere protein recoveries were less than 90%, an analysis of the devicesindicated that the retentive layer of membrane had been damaged. In somecases the damage occurred due to contact between the membrane and thecore pin. In some cases the membrane coupons were improperly aligned onthe panels to the attachment process, which allowed protein leakage tooccur. In other cases the damage may have occurred due to manualhandling at the site where the membrane was attached or at the sitewhere overmolding was done.

FIG. 16 clearly shows that the device filters water much faster than theMicrocon® type device. The data also show that the device spends filterssample fluids to a known dead stop volume which helps to prevent dry outconditions, whereas the Microcon device allows sample volumes to befiltered to dryness, which in most cases is undesirable.

FIGS. 17 through 22 present the results of tests that were done ondevices that were made using pilot molds for the panels that consistedof one 4-cavity mold, which was used to simulate as closely as possiblea production molding tool. Similarly the pilot overmold tool consistedof one 2-cavity mold, which was used to simulate as closely as possiblethe production tooling with the exception of the panel handling system.For pilot operations the panels were manually loaded into the overmoldtool by hand, whereas in production operations panels would be loaded into the production molds using robotics.

These figures present the results of performance testing on samples ofthe devices that were made using pilot molds. Tests were done on devicesthat represent the five membranes-device configurations that are ofinterest. The testing consisted of: a) air leak tests, b) percentage ofprotein passage, c) percentage of proteins recovered, d) dead-stopvolume after 10 minutes of spinning at 14,000 Gs, e) total volume ofsample recovered, and f) housing burst pressure.

TABLE 2 Average StdDev. 3K 10K 30K 50K 100K 3K 10K 30K 50K 100K LeakRate cc/min 0.131 0.105 0.097 0.092 0.092 0.056 0.055 0.056 0.061 0.052% Protein Passage 0.163 0.223 2.080 3.472 1.508 0.186 0.358 1.345 0.7980.607 % Protein Recovery 97.8 95.4 96.2 92.8 90.0 1.250 1.441 1.3682.016 2.498 Dead Stop Volume 0.069 0.034 0.023 0.019 0.017 0.003 0.0020.001 0.001 0.001 % Total Volume 98.6 98.5 98.7 98.6 98.7 0.446 0.2650.628 0.464 0.375 Recovered Housing Burst 298.4 297.0 286.0 275.3 292.925.279 15.079 17.442 15.851 12.170

The membranes of interest were attached to the pilot panels and werelater over molded into devices. These devices were then tested for sealintegrity using a pressure-leak tester. Air pressure of 30 psi wasapplied to the devices and the leakage rate of air across the membranewas measured. FIG. 17 shows that all five of the differentmembrane-panel configurations demonstrated acceptablemembrane-attachment integrity. These panels were then carefully packagedand shipped to the molding facility for final overmolding into devices.

FIG. 17 also shows that the typical values of airflow leakage were lessthan 0.35 cubic centimeters (cc) per minute. This limit, for acceptingand rejecting parts was determined by evaluating the results of morethan n=50 devices that were determined to be acceptable, as measured byacceptable protein retention performance. This limit represents theupper control limit (three standard deviations above the average value)of the population of samples.

FIG. 18 shows that the overmolding process successfully bonded the twopanels into a single device in a manner that demonstrated acceptablepressure integrity. The data show that the average housing-burstpressure for n=100 devices that were made using all five of themembranes of interests or greater than 250 psi. At least n=20 deviceswere tested using each of the membrane-panel configurations (3 kDa MWCO,10 kDa MWCO, 30 kDa MWCO, 50 kDa MWCO & 100 kDa MWCO).

FIG. 19 shows that the levels of average protein passage (proteins thatshould not pass) were less than 5%, which is considered to be anacceptable level of performance. At least n=24 devices were tested usingeach of the five different membrane-panel configurations.

FIG. 20 shows that the average protein that was recovered was greaterthan 90% for the devices that were tested. At least n=24 devices weretested using each of the five different membrane-panel configurations.

FIG. 21 shows that the average total volume of sample that was recoveredfor these devices was greater than 98% when the reverse spin procedurewas used. At least n=24 devices were tested using each of the fivedifferent membrane-panel configurations.

FIG. 22 shows the results of the average dead-stop volume that wasmeasured when these devices were centrifuged for 10 minutes time. Theresults show that three of the membrane-device configurations (30 kDaMWCO, 50 kDa MWCO & 100 kDa MWCO) were able to achieve dead-stop volumeswithin 10 minutes. Two of the membrane-device configurations (3 kDa MWCO& 10 kDa MWCO) did not filter down to their dead-stop volumes within 10minutes. As expected these two membranes have a much tighter membranestructure and the other three membranes, and require more time to filterthe same volume of fluid. Devices that are made using 3K MWCO membraneswill typically require at least 16 minutes to achieve dead stop volumesof 20 μL when filtering typical protein solutions. Devices that are madeusing 10K MWCO membranes will typically require at least 12 minutes toachieve dead stop volumes of 20 μL when filtering typical proteinsolutions.

1. A centrifugal filtration assembly, comprising: a filtration device and at least one filtration device holder; said filtration device comprising a sample reservoir, a pair of spaced membranes each supported by an underdrain support comprising a series of channels, grooves or surface textures to capture filtrate, a plurality of drain holes for draining said filtrate into said vial, and a retentate chamber, said filtration device having an external diameter; said device holder comprising a housing having an open end and a first internal diameter larger than said external diameter of said filtration device, said device holder having a shoulder within said housing spaced from said open end and defining a second internal diameter smaller than said external diameter of said filtration device; wherein said filtration device is insertable into said device holder in a spin mode whereby filtrate from said filtration device flows from said drain holes into said device holder housing, and in a reverse spin mode whereby retentate in said retentate chamber flows into said device holder housing, and wherein the extent of the insertion of said filtration device into said device holder in either said spin mode or said reverse spin mode is limited by said shoulder.
 2. The centrifugal filtration assembly of claim 1, wherein said device holder further comprises a cap for closing said open end.
 3. The centrifugal filtration assembly of claim 2, wherein said filtration device has an open end, and wherein said cap is shaped to close said open end.
 4. The centrifugal filtration assembly of claim 1, wherein each said membrane is oriented at an angle of greater than about 0° and less than about 5° from the vertical centerline of said filtration device.
 5. The centrifugal filtration assembly of claim 1, wherein each said membrane is oriented at an angle of about 3° from the vertical centerline of said filtration device.
 6. The centrifugal filtration assembly of claim 1, wherein each of said drain holes has a draft angle of about 5°.
 7. The centrifugal filtration assembly of claim 1, wherein said retentate chamber comprises a dead volume.
 8. A centrifugal filtration assembly, comprising: a filtration device and at least one filtration device holder; said filtration device comprising a sample reservoir, a pair of spaced membranes each supported by an underdrain support comprising a series of channels, grooves or surface textures to capture filtrate, a plurality of drain holes for draining said filtrate into said vial, and a retentate chamber, said filtration device having an external diameter; said device holder comprising a housing having an open end and an internal diameter; wherein said filtration device is insertable into said device holder in a spin mode whereby filtrate from said filtration device flows from said drain holes into said device holder housing, and in a reverse spin mode whereby retentate in said retentate chamber flows into said device holder housing, and wherein the extent of the insertion of said filtration device into said device holder in delimited by the relative lengths of said external diameter and internal diameter.
 9. A method of filtering a sample, comprising: providing a filtration device and first and second filtration device holders; said filtration device comprising a sample reservoir, a pair of spaced membranes each supported by an underdrain support comprising a series of channels, grooves or surface textures to capture filtrate, a plurality of drain holes for draining said filtrate into said vial, and a retentate chamber, said filtration device having an external diameter; each said device holder comprising a housing having an open end and a first internal diameter larger than said external diameter of said filtration device, said device holder having a shoulder within said housing spaced from said open end and defining a second internal diameter smaller than said external diameter of said filtration device; introducing sample into said sample reservoir; inserting said filtration device into said first device holder in an orientation wherein said drain holes are in fluid communication with said device holder housing; applying centrifugal force to said sample to filter said sample and cause filtrate to flow from said drain holes into said first device holder housing and to accumulate retentate in said retentate chamber; removing said filtration device from said first device holder; insert said filtration device into said second device holder in an orientation wherein said retentate chamber is in fluid communication with said second device holder housing; applying centrifugal force to said retentate in said retentate chamber to transfer said retentate into said second device holder housing; and removing said filtration device from said second device holder. 